Positive and negative electrode plates of batteries are constructed by densely coating core members, which will function as current collectors of the batteries, with respective mixture pastes of positive and negative active materials. Various materials have been used for the core member, such as sintered substrates (e.g. Japanese Patent Laid-Open Publication No. Hei. 4-165006), foamed porous metal substrates (e.g. Japanese Patent Publication No. Sho. 57-39317), punched, corrugated substrates (e.g. Japanese Patent Laid-Open Publication No. Hei. 7-130370), punched metal substrates, and expanded metal substrates (e.g. Japanese Patent Laid-Open Publication Nos. Hei. 3-204126 and Hei. 7-335208).
Core members are desired to have such properties as a high current collecting efficiency, an excellent active material-retaining capability, a small volume for a higher energy density, and a structure that allows a good flow of ions and gas that are generated from liquid electrolyte or through chemical reactions inside the battery. In addition to these requirements, it is desired that the core members be manufactured on a low-cost, mass production basis. None of the conventional core members mentioned above has yet satisfied all of the requirements for the properties, costs, and mass producibility in a well-balanced manner.
That is, the prior art core members mentioned above each have some favorable properties of their own, but they are all produced through a relatively large number of process steps, and therefore they have the common problems of high production costs and poor mass producibility. The characteristics of each of the prior art core members will be described below. Sintered substrates contribute to good current collecting properties when constructed as electrode plates and they are also excellent in the capability of retaining the active material. On the other hand, they are not suitable for achieving a higher energy density of electrode plates, because of the low proportion of pores which retain the active material.
Foamed porous metal substrates have an extremely high porosity and a high specific surface area and they exhibit outstanding large current discharge characteristics as compared to other core members. One drawback of this type of substrates is that short circuiting tends to occur resulting from burrs on the metal. Another reason for the frequent short circuiting is that the electrode plate obtained by coating a foamed porous metal substrate with a mixture paste and by rolling same includes a portion on its surface where metal is exposed. The most significant drawback, however, of foamed porous metal substrates is high costs.
Punched and embossed substrates are apt to stretch when subjected to tension or rolling, and so they can readily be deformed. Another disadvantage is that short circuiting tends to occur because of the burrs which are formed when punching the substrates and which are left intentionally so as to enhance the capability of retaining the active material.
Punched metal substrates and expanded metal substrates are of flat shape and the active materials contained in the mixture paste which is coated on both sides of the substrates are merely bound together by a binder in the mixture paste. Thus their active material-retaining capability tends to be insufficient. Particularly when they are formed into a spiral electrode plate assembly, the active materials can easily peel off, thereby shortening the battery life. Also, because these core members are of flat shape, the resultant electrode plates are inherently poor in the current collecting properties in their thickness direction.
Researches have been carried out to deal with the above problems and to develop a core member that can satisfy all the requirements for various properties, costs, and producibility in a well-balanced manner. For example, with regard to the expanded metal substrates, Japanese Patent Laid-Open Publication No. Hei. 3-204126 proposes a method of producing an expanded mesh sheet, which is an improved version of the method of producing an expanded mesh sheet disclosed in Japanese Patent Publication No. Sho. 60-29573 (whereby a mesh sheet is obtained in which a multiplicity of strings form a lattice-like pattern with the knots arranged in a zigzag fashion).
One requirement regarding such expanded mesh sheet was to make it thinner so as to achieve a higher energy density. Thus various proposals have been made, such as a method of producing an expanded mesh sheet (Japanese Patent Laid-Open Publication No. Hei. 11-260373), whereby a metal lath of a fine mesh including plain parts is obtained by a reciprocating type slit forming method. Also, a non-aqueous electrolyte battery (proposed in Japanese Patent Laid-Open Publication No. Hei. 11-260418) employs an expanded metal substrate, which is cut out from a band-like, metal lath sheet of a thickness of 0.1 mm or lower. The metal lath sheet has plain parts along its lengthwise direction at given locations in its widthwise direction, and these plain parts correspond to the lead connecting portions of the cut-out substrates. However, these expanded mesh sheets (or expanded metal substrates) can only provide two-dimensional core members, and therefore the resultant core members are inferior in the current collecting properties in the thickness direction.
A possible option to improve the current collecting properties in the thickness direction of such two-dimensional core members (or, in other words, to achieve three-dimensional current collecting properties) is to add carbon, as a conductive agent, to the mixture paste that chiefly includes an active material. However, this will not work for a nickel hydride rechargeable battery, for example, which is one of the batteries that require three-dimensional current collecting properties, because, if carbon is added as a conductive agent to the positive electrode mixture of such battery, it will be oxidized during charging, producing carbon dioxide ions. Thereby the internal resistance of the battery will increase, and also the active material will easily peel off with the expansion and shrinkage of the battery as it is charged and discharged. As a result, the battery life will be shortened. Because of these reasons, the foamed porous metal substrate has been used for the positive electrode plate of nickel hydride rechargeable batteries. However, the foamed porous metal substrate has the drawback of high costs as mentioned above, and also, it is difficult to provide plain parts, which will be required for bonding a current collector to the resultant electrode plate or electrode plate assembly, in the substrate. Moreover, an attempt to make the substrate thinner will result in a decrease in the porosity, which is contrary to the desire to achieve a high capacity density.
In light of the above-described prior art problems, it is an object of the present invention to provide a thin battery electrode plate, which has excellent properties such as a high strength against the tension applied during a rolling process and a low risk of inner short-circuiting, and which can be mass produced at low cost, but achieve three-dimensional current collecting properties and a method of producing such battery electrode plate.